When I attack someone then my troops are dying but his troops only wounded what is the reason?

You must research Deadly Arms at the bottom of the Basic Military research, Carbon Fiber Bones and Viral Spores at the bottom of Zones Conflict research, and Super Healing at the bottom of Improved Medical Facilities. Those can affect how many of the enemy's troops die and how many of yours can be saved from death and be wounded instead. The player you were fighting probably had some variation of the research I provided complete in their institute.

Good luck.

-Puff the Magic Dragon